Project Summary/Abstract CMS recently began paying for total knee replacement surgery (arthroplasty; TKA) performed in hospital outpatient departments (HOPDs) and is considering reimbursing for HOPD total hip arthroplasty (THA). Many observers expect Medicare payment for TKA and THA in ambulatory surgery centers (ASCs) to follow. Currently, total joint replacements are being performed on commercially insured patients in HOPDs and in a growing number of ASCs. However, despite claims by physician-owners of ASCs that outpatient total joint replacement is appropriate and safe for a large portion of candidates, evidence to date is based on a limited set of studies using very small samples of patients treated in HOPDs. As ASCs have steadily expanded their business and taken on increasingly complex surgeries, some experts fear they overlook high-risk health problems. Yet it is possible that comparable outcomes can be achieved with less reliance on traditional health system resources. The majority of joint replacement recipients in the U.S. are Medicare beneficiaries, and outcomes data on outpatient TKA in this population will not be available for some time. In the interim, we propose to compare TKA and THA outcomes and payments across inpatient, HOPD, and ASC settings using a large sample of commercially- insured patients. We will identify near elderly patients who underwent TKA or THA in an inpatient or outpatient ASC setting during 2014-2019 and who were continuously enrolled for 90 days following surgery. We will select matched samples of patients across inpatient, HOPD and ASC settings using propensity score techniques, matching with risk scores generated from the HHS Hierarchical Condition Categories risk adjustment model which was designed under the ACA for application to the commercially-insured population. We will identify cases of related 30-day and 90-day readmission, and cases within 90-days of revision surgery, surgical site infection, deep-vein thrombosis, pulmonary embolism, and dislocation/loosening/breakage of prosthesis. We will test differences across sites using statistical techniques. Finally, we will measure the relative cost of episodes of care including related expenditures incurred during the 90-day post-inpatient stay or ASC encounter. This project will enlighten CMS on potential health consequences of further transitions to outpatient surgery. For providers, it will identify areas of potential patient safety lapses, and targets for interventions. Finally, it will facilitate patient engagement in their care by informing ?gray area? candidates for outpatient joint replacement in making wiser decisions about where to undergo surgery.